Chitosan, a natural polysaccharide, is nontoxic, lightweight and biodegradable, which exhibits a great potential for the emerging flexible and "green" electronic applications. Here, lightweight flexible aluminum-zinc-oxide (AZO) thin-film transistors (TFTs) are fabricated using chitosan biopolymer as self-supporting layer. This kind of biopolymer electrolyte can provide a strong electric-double-layer effect, which leads to a large capacitance with lower energy consumption. With the low-cost indium-free AZO deposited onto the chitosan film as the coplanar gate and source/channel/drain electrodes, the transistor shows a moderate on/off ratio of similar to 10(4), a relatively ideal field-effect mobility of 0.3 cm(2)/Vs and a moderate sub-threshold swing of 0.65 V/dec. Moreover, logic "AND" function is realized in the flexible device with two coplanar gates as the input terminals. Such chitosangated flexible TFT devices can provide promising candidates for the next generation wearable and "green" electronics.